Avalanchas en un fosa de socavación alrededor de un cilindro hincado en arena: un mecanismo relevante de socavación

Abstract

The aperiodic dynamically rich horseshoe vortex system at the base of a sediment-embedded cylinder in a turbulent boundary layer is the main agent of scour, dislodging and transporting sediment particles, and producing sand slides at the sides of the scour hole. While the sediment transport is linked to the enhanced bed shear stresses induced by the horseshoe vortices, the precise scour mechanisms are not fully understood. Physical scouring mechanisms are not included in scour-depth equations. Therefore, a conceptual framework linking the sediment dynamics at the intra scour hole scale with the macroscale properties of the flow is missing. In this work, the sand slides developed in a scour hole around a cylinder in a sand bed were experimentally investigated. Experiments were conducted in a laboratory flume with constant discharge and clear-water conditions, with flow intensities - corresponding to the ratio between the section averaged flow velocity and the critical velocity for inception of sediment motion- equal to 0.68, 0.78, and 0.84, until advanced stages of scour. Sand slides were recorded in a non-intrusive manner during a representative segment of time with a frequency of 30 fps using a video camera placed within a plexiglass cylinder of 15 cm in diameter. Through a Proper Orthogonal Decomposition of the image sequences, two types of sand slides were identified: Type I was a short and long advancing sand slide, while Type II was an elongated and short advancing sand slide. In time, sand slides occurred intermittently in an aperiodic manner resembling the behavior of the horseshoe vortex. The frequency of occurrence of sand slides decreased, and magnitude of sand slides increased with time. The product of frequency and magnitude, which expresses the rate of erosion caused by sand slides, decreased in time, evidencing a reduction of erosion towards equilibrium conditions. Interestingly, the sand slide Strouhal number correlated with the dimensionless flow work, W* needed to deepen the scour hole depth, Z* in a given amount, ∂Z*/∂W*. Sand slides of Type I occurred for low values of ∂Z*/∂W*, while sand slides of Type II occurred for high values of ∂Z*/∂W*. Apart from the well-known sediment dislodging, rolling and sliding due to the enhanced bed shear stresses, our results show that sand slides are an important scour mechanism, contributing to produce about 60% to the scoured volume, and decrease to 40% when flow intensity was increased.